Cyclodecapentaene

Cyclodecapentaene or 0nnulene is an annulene with molecular formula C₁₀H₁₀. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because various types of ring strain destabilize an all-planar geometry.

Conformation, strain, and non-aromaticity

Although not aromatic itself, 0nnulene can transition between different con­forma­tional isomers through aromatic or quasi­aromatic excited states, such that its con­forma­tional iso­mer­ism is fixed only at extreme cryogenic temperatures. Under­stand­ing the com­posi­tion and react­ivity of these mix­tures com­put­ation­ally has proven difficult, because a large number of conformations all minimize the energy locally.

The all- ''cis'' isomer ( 1), a fully convex decagon, would have bond angles of 144°, which creates large amounts of angle strain relative to the ideal 120° in sp² atomic hybridization. Instead, the all-''cis'' isomer adopts a planar boat-like conformation ( 2) to relieve the angle strain, although it, too, is less stable than the next planar isomer, ''trans'',''cis'',''trans'',''cis'',''cis''- 0nnulene ( 3). Yet even isomer (3) is unstable, suffering from steric repulsion between the two internal hydrogen atoms, and tends to distort into the perimeter of two fused circles, one larger and the other smaller, as in azulene. The nonplanar ''trans'',­''cis'',­''cis'',­''cis'',­''cis'' isomer is the most stable of all possible isomers, although it is unclear whether it too has a boat-like configuration as in conformer ( 4), or the "heart" configuration produced if one internal hydrogen in conformer ( 3) were flipped inside-out.

Synthesis

Cyclodecapentaene can undergo an electrocyclic rearrangement to or from dihydronaphthalene. Photolysis of the latter generates 0nnulene, but it quickly reverts to the reactant, even at cryogenic temperatures.

Aromatic derivatives

Aromaticity can be induced in compounds having a 0nnulene-type core if planarity is forcibly imposed by other substituents. Two methods to do so are known.

One method is to formally replace two hydrogen atoms by a methylene bridge (); this gives the planar bicyclic 1,6-methano­ 0nnulene ( 5). Indeed, 1,6-methano­ 0nnulene has no bond length alternation in its X-ray structure and signs of a telltale diamagnetic ring current in its NMR spectrum. Likewise, a tricyclic methine bridge gives an aromatic structure ( 6) similar to the stable oxonium ion oxatriquinacene.

When de­proton­ated to form the anion this type of compound is even more stabilized. The central carbanion enhances the molecule's planarity and the number of resonance structures that can be drawn is extended to 7, including two resonance forms with a complete benzene ring. Computational chemistry suggests a tricyclic 0nnulene derivative with an annulated benzene ring and a full set of cyano substituents ( 7) would be one of the most acidic compounds known, with a computed p''K''_a in DMSO of −30.4 (compared to for instance −20 for magic acid).

The other method is to further remove hydrogens and develop triple bonds or cyclopropanes along the ring. Thus com­puta­tional studies suggest that cyclo­deca­tetraene­yne is (although formally a 12-π system) planar and aromatic, as is bicyclo .1.0ndeca-1,3,7,9-tetraen-5-yne. Predicting the aroma­ticity of these compounds is not always obvious: the polycyclic hydrocarbon tetra­dihydro­naphtho­ 0nnulene, in which a valence isomer of 0nnulene is fused to two naphthalenes, does not exhibit aromaticity inside the central 10-π ring.

Other related compounds

* Azulene is also a 10 π-electron system in which aromaticity is maintained by direct trans­annular bonding to form a fused 7–5 bicyclic molecule.
* Cyclodecatetraene is a stable, non-aromatic 8 π-electron system with no ring strain.

References

{{Annulenes

Annulenes
Fully conjugated nonaromatic rings
Ten-membered rings